1. Field of Invention
This invention relates to a method for the production of single crystal material and, in particular, to an improved continuous method for the production of single crystal silicon.
2. Brief Statement of the Prior Art
Single crystal silicon is the basic substrate used for virtually all semiconductor devices. Other single crystal materials which are also finding applications in semiconductor devices are germanium and gallium arsenide. These materials are synthetically produced in a purified and perfect, single crystal state. The method traditionally used for such production has been the Czochralski method. In the Czochralski method, polycrystalline material, such as silicon, is melted and maintained in a molten state in a quartz crucible. The quartz crucible is supported by a graphite cup that is mounted in a heated furnace. A seed crystal of silicon is dipped in the molten silicon and is slowly withdrawn, forming a cylindrical boule of single crystal silicon. The boule and crucible are rotated counter-rotationally to promote uniformity of silicon properties and distribution of impurities and dopant additives within the silicon.
The Czochralski process is conducted batchwise and inherent limitations of batchwise processing cause, or promote, variations in properties and composition of the silicon boule. Dopants such as phosphorus or boron are usually added to the silicon melt to impart desirable semiconductor properties to the silicon wafers which are sliced from the cylindrical silicon boule. The dopants tend to concentrate in the molten pool as the pool is deplenished by the forming of the boule. Additionally, oxygen is introduced into the molten pool from reaction of the silicon melt with the surfaces of the quartz crucible. As the level of the molten pool decreases during the batch processing the wetted surface area decreases, resulting in a continuous decrease in oxygen concentration in the melt.
These and related inherent limitations in the Czochralski process have led to the improved process which is described in U.S. Pat. No. 4,659,421. In the improved process, the molten pool is maintained in a stationary crucible; a replenishment zone is established in the stationary crucible, laterally separated from a crystal growth zone, where the boule is formed. The molten silicon is mechanically pumped from the replenishment zone to the growth zone by rotating a feed rod of polycrystalline silicon, thereby stirring the molten liquid and inducing it to flow towards the crystal growth zone. The major advance of the improved Czochralski process was maintaining of the molten silicon in a shallow pool, thereby reducing the variation of dopant and oxygen contents in the cylindrical boule withdrawn from the pool.
The aforementioned improved process has some limitations. One limitation is that the improved process requires a completely redesigned furnace which utilizes none of the components of the traditional Czochralski process furnaces. The improved process requires that the feed material be supplied as a solid rod to stir the molten silicon pool, which is maintained in a stationary crucible. This requirement precludes the use of granular polycrystalline feed material. While the improved process produces silicon ingots of improved uniformity, a substantial investment in new process equipment is required for its practice. Also, the improved process does not anneal or thermal condition the single crystal boule.